Abnormalities caused by carbohydrate alterations in Ibeta6-N-acetylglucosaminyltransferase-deficient mice

Comprehensive single-cell transcriptional profiling defines shared and unique epithelial injury responses during kidney fibrosis

The underlying cellular events driving kidney fibrogenesis and metabolic dysfunction are incompletely understood. Here, we employed single-cell combinatorial indexing RNA sequencing to analyze 24 mouse kidneys from two fibrosis models. We profiled 309,666 cells in one experiment, representing 50 cell types/states encompassing epithelial, endothelial, immune, and stromal populations. Single-cell analysis identified diverse injury states of the proximal tubule, including two distinct early-phase populations with dysregulated lipid and amino acid metabolism, respectively. Lipid metabolism was defective in the chronic phase but was transiently activated in the very early stages of ischemia-induced injury, where we discovered increased lipid deposition and increased fatty acid β-oxidation. Perilipin 2 was identified as a surface marker of intracellular lipid droplets, and its knockdown in vitro disrupted cell energy state maintenance during lipid accumulation. Surveying epithelial cells across nephron segments identified shared and unique injury responses. Stromal cells exhibited high heterogeneity and contributed to fibrogenesis by epithelial-stromal crosstalk.

Targeted disruption of Rab1a causes early embryonic lethality

Guanosine nucleotide diphosphate (GDP) dissociation inhibitor 2 (GDI2) regulates the GDP/guanosine triphosphate (GTP) exchange reaction of Rab proteins by inhibiting the dissociation of GDP and the subsequent binding of GTP. The present study aimed to determine the function of Rab1a in vivo, and thus generated mice with a trapped Rab1a gene. It was demonstrated that Rab1a is essential for embryonic development. It was also found that one functional Rab1a allele was sufficient for development in a heterozygous murine embryo, whereas a double mutant led to embryonic lethality. The dissection of uteri on embryonic day (E)10.5‑14.5 yielded no homozygous embryos, indicating that homozygotes die between E10.5 to E11.5. The gene trap construct contains a β‑galactosidase/neomycin reporter gene, allowing for heterozygotes to be stained for β‑galactosidase to determine the tissue‑specific expression of Rab1a. Rab1a was found to be highly expressed in the small intestine of both adult mice and embryos, although its expression levels were low in the brains of embryos. Moreover, there was no significant change in cytokine production and survival in wild‑type and heterozygous Rab1a+/‑ mice following a challenge with lipopolysaccharide. On the whole, the present study demonstrates that the disruption of the Rab1a gene causes embryonic lethality and homozygotes die between E10.5 and E11.5, suggesting that Rab1a is essential for the early development of mouse embryos.

Deletion at the GCNT2 Locus Causes Autosomal Recessive Congenital Cataracts

PURPOSE

The aim of this study is to identify the molecular basis of autosomal recessive congenital cataracts (arCC) in a large consanguineous pedigree.

METHODS

All participating individuals underwent a detailed ophthalmic examination. Each patient's medical history, particularly of cataracts and other ocular abnormalities, was compiled from available medical records and interviews with family elders. Blood samples were donated by all participating family members and used to extract genomic DNA. Genetic analysis was performed to rule out linkage to known arCC loci and genes. Whole-exome sequencing libraries were prepared and paired-end sequenced. A large deletion was found that segregated with arCC in the family, and chromosome walking was conducted to estimate the proximal and distal boundaries of the deletion mutation.

RESULTS

Exclusion and linkage analysis suggested linkage to a region of chromosome 6p24 harboring GCNT2 (glucosaminyl (N-acetyl) transferase 2) with a two-point logarithm of odds score of 5.78. PCR amplifications of the coding exons of GCNT2 failed in individuals with arCC, and whole-exome data analysis revealed a large deletion on chromosome 6p in the region harboring GCNT2. Chromosomal walking using multiple primer pairs delineated the extent of the deletion to approximately 190 kb. Interestingly, a failure to amplify a junctional fragment of the deletion break strongly suggests an insertion in addition to the large deletion.

CONCLUSION

Here, we report a novel insertion/deletion mutation at the GCNT2 locus that is responsible for congenital cataracts in a large consanguineous family.

Embryoglycan: a highly branched poly-N-acetyllactosamine in pluripotent stem cells and early embryonic cells

Embryonal carcinoma cells, stem cells of teratocarcinomas, are pluripotent stem cells and also prototypes of embryonic stem cells. Embryonal carcinoma cells contain large amounts of a highly branched poly-N-acetyllactosamine called embryoglycan, which has a molecular weight of approximately 10,000 or greater, and is asparagine-linked. This glycan was found by analyses of fucose-labeled glycopeptides, and its characteristics were established by biochemical analyses. The content of embryoglycan progressively decreases during the in vitro differentiation of embryonal carcinoma cells. Embryoglycan is also abundant in mouse embryonic stem cells and preimplantation mouse embryos, and decreases during embryogenesis. Embryoglycan carries a number of carbohydrate markers of murine pluripotent stem cells. Lewis x markers, such as SSEA-1, 4C9 antigen, and binding sites for Lotus tetragonolobus agglutinin are of particular importance. 4C9 antigenicity requires clustering of Lewis x, best accomplished by poly-N-acetyllactosamine branching, whereas SSEA-1 does not. Although in vivo evidence is lacking, these epitopes have been suggested to participate in cell-to-cell and cell-to-substratum adhesion. Other markers on embryoglycan include α-galactosyl antigens such as ECMA-2, and binding sites for Dolichos biflorus agglutinin, the epitope of which is considered to be identical to Sd^a antigen, namely, GalNAcβ1-4(NeuAcα2-3)Galβ1-4GlcNAc. While embryoglycan is also present in human teratocarcinoma cells, the carbohydrate markers characterized in human pluripotent stem cells to date are largely carried by glycolipids and keratan sulfate. Information on embryoglycan and markers carried by it may assist in the development of new markers of human pluripotent stem cells and their progenies.

N-linked polylactosamine glycan synthesis is regulated by co-expression of β3GnT2 and GCNT2

Poly-N-acetyllactosamine (PLN) is a unique glycan composed of repeating units of the common disaccharide (Galβ1,4-GlcNAcβ1,3)n . The expression of PLN on glycoprotein core structures minimally requires enzyme activities for β1,4-galactosyltransferase (β4GalT) and β1,3-N-acetylglucosminyltransferase (β3GnT). Because β4GalTs are ubiquitous in most cells, PLN expression is generally ascribed to the tissue-specific transcription of eight known β3GnT genes in mice. In the olfactory epithelium (OE), β3GnT2 regulates expression of extended PLN chains that are essential for axon guidance and neuronal survival. N-glycan branching and core composition, however, can also modulate the extent of PLN modification. Here, we show for the first time that the β1,6-branching glycosyltransferase GCNT2 (formerly known as IGnT) is expressed at high levels specifically in the OE and other sensory ganglia. Postnatally, GCNT2 is maintained in mature olfactory neurons that co-express β3GnT2 and PLN. This highly specific co-expression suggests that GCNT2 and β3GnT2 function cooperatively in PLN synthesis. In support of this, β3GnT2 and GCNT2 co-transfection in HEK293T cells results in high levels of PLN expression on the cell surface and on adenylyl cyclase 3, a major carrier of PLN glycans in the OE. These data clearly suggest that GCNT2 functions in vivo together with β3GnT2 to determine PLN levels in olfactory neurons by regulating β1,6-branches that promote PLN extension.

Molecular genetics of the blood group I system and the regulation of I antigen expression during erythropoiesis and granulopoiesis

PURPOSE OF REVIEW

The molecular genetics of the blood group I system and the regulation mechanism for I antigen expression in postnatal red blood cells are intriguing. This review summarizes their elucidation and recent findings.

RECENT FINDINGS

Accumulating data from the molecular analysis of individuals with the adult i phenotype supports the proposed molecular genetic mechanism for the partial association of the adult i phenotype with congenital cataracts. Recent investigations have shown that the regulation of I antigen formation during erythropoiesis is determined by transcription factor CCAAT/enhancer binding protein-α (C/EBPα) and the phosphorylation status of C/EBPα Ser-21 residue.

SUMMARY

The human I locus is organized such that it has an uncommon genetic architecture and expresses three different I transcript forms. The results obtained from molecular analysis of two adult i groups, with and without congenital cataracts, demonstrate that the molecular background accounts for the partial association between these two traits and suggest that an I gene defect may lead directly to the development of congenital cataracts. Analysis of the regulation for I antigen expression shows that the regulation during erythropoiesis and granulopoiesis share a common mechanism, with dephosphorylation of the Ser-21 residue on C/EBPα playing the critical role.

Cat-Map: putting cataract on the map

Lens opacities, or cataract(s), may be inherited as a classic Mendelian disorder usually with early-onset or, more commonly, acquired with age as a multi-factorial or complex trait. Many genetic forms of cataract have been described in mice and other animal models. Considerable progress has been made in mapping and identifying the genes and mutations responsible for inherited forms of cataract, and genetic determinants of age-related cataract are beginning to be discovered. To provide a convenient and accurate summary of current information focused on the increasing genetic complexity of Mendelian and age-related cataract we have created an online chromosome map and reference database for cataract in humans and mice (Cat-Map).

High-sensitivity O-glycomic analysis of mice deficient in core 2 {beta}1,6-N-acetylglucosaminyltransferases

Core 2 β1,6-N-acetylglucosaminyltransferase (C2GnT), which exists in three isoforms, C2GnT1, C2GnT2 and C2GnT3, is one of the key enzymes in the O-glycan biosynthetic pathway. These isoenzymes produce core 2 O-glycans and have been correlated with the biosynthesis of core 4 O-glycans and I-branches. Previously, we have reported mice with single and multiple deficiencies of C2GnT isoenzyme(s) and have evaluated the biological and structural consequences of the loss of core 2 function. We now present more comprehensive O-glycomic analyses of neutral and sialylated glycans expressed in the colon, small intestine, stomach, kidney, thyroid/trachea and thymus of wild-type, C2GnT2 and C2GnT3 single knockouts and the C2GnT1-3 triple knockout mice. Very high-quality data have emerged from our mass spectrometry techniques with the capability of detecting O-glycans up to at least 3500 Da. We were able to unambiguously elucidate the types of O-glycan core, branching location and residue linkages, which allowed us to exhaustively characterize structural changes in the knockout tissues. The C2GnT2 knockout mice suffered a major loss of core 2 O-glycans as well as glycans with I-branches on core 1 antennae especially in the stomach and the colon. In contrast, core 2 O-glycans still dominated the O-glycomic profile of most tissues in the C2GnT3 knockout mice. Analysis of the C2GnT triple knockout mice revealed a complete loss of both core 2 O-glycans and branched core 1 antennae, confirming that the three known isoenzymes are entirely responsible for producing these structures. Unexpectedly, O-linked mannosyl glycans are upregulated in the triple deficient stomach. In addition, our studies have revealed an interesting terminal structure detected on O-glycans of the colon tissues that is similar to the RM2 antigen from glycolipids.

Regulation of glycan structures in animal tissues: transcript profiling of glycan-related genes

Glycan structures covalently attached to proteins and lipids play numerous roles in mammalian cells, including protein folding, targeting, recognition, and adhesion at the molecular or cellular level. Regulating the abundance of glycan structures on cellular glycoproteins and glycolipids is a complex process that depends on numerous factors. Most models for glycan regulation hypothesize that transcriptional control of the enzymes involved in glycan synthesis, modification, and catabolism determines glycan abundance and diversity. However, few broad-based studies have examined correlations between glycan structures and transcripts encoding the relevant biosynthetic and catabolic enzymes. Low transcript abundance for many glycan-related genes has hampered broad-based transcript profiling for comparison with glycan structural data. In an effort to facilitate comparison with glycan structural data and to identify the molecular basis of alterations in glycan structures, we have developed a medium-throughput quantitative real time reverse transcriptase-PCR platform for the analysis of transcripts encoding glycan-related enzymes and proteins in mouse tissues and cells. The method employs a comprehensive list of >700 genes, including enzymes involved in sugar-nucleotide biosynthesis, transporters, glycan extension, modification, recognition, catabolism, and numerous glycosylated core proteins. Comparison with parallel microarray analyses indicates a significantly greater sensitivity and dynamic range for our quantitative real time reverse transcriptase-PCR approach, particularly for the numerous low abundance glycan-related enzymes. Mapping of the genes and transcript levels to their respective biosynthetic pathway steps allowed a comparison with glycan structural data and provides support for a model where many, but not all, changes in glycan abundance result from alterations in transcript expression of corresponding biosynthetic enzymes.

Embryonic stem cells deficient in I beta1,6-N-acetylglucosaminyltransferase exhibit reduced expression of embryoglycan and the loss of a Lewis X antigen, 4C9

Embryoglycan is a class of branched high-molecular-weight poly-N-acetyllactosamines characteristically expressed in early embryonic cells and has been shown to be involved in the intercellular adhesion of early embryonic cells in vitro. Branching of poly-N-acetyllactosamine chains is performed by beta1,6-N-acetylglucosaminylation of the galactosyl residue. We previously knocked out the gene encoding I beta1, 6-N-acetylglucosaminyltransferase (IGnT), and the resultant deficient mice were born without any abnormality, although the mice exhibited various deficits in later life. In the present investigation, we produced embryonic stem (ES) cells from IGnT-deficient embryos. The mutant ES cells exhibited a reduced capability in embryoglycan synthesis. Thus, IGnT is a major enzyme involved in the branching of poly-N-acetyllactosamine chains in embryoglycan. Since ES cells are equivalent to multipotential cells of the embryonic ectoderm in early postimplantation embryos, this result indicates that an abundance of embryoglycan in these cells is not essential for normal embryogenesis. The IGnT-deficient ES cells continued to express SSEA-1, but lacked the expression of 4C9 antigen, although the epitope of 4C9 antigen was confirmed to be Lewis X by a transfection experiment. The result establishes the distinct nature of 4C9 antigenicity, which requires either Lewis X epitope on I-branch or clustering of Lewis X epitope, best accomplished by poly-N-acetyllactosamine branching. Alpha6-integrin was newly identified as a carrier of embryoglycan. The IGnT-deficient ES cells adhered to dishes coated with laminin, which is a ligand for alpha6-integrin, significantly less than wild-type ES cells, raising the possibility that embryoglycan in ES cells enhances alpha6-integrin-dependent adhesion in vitro.